# std::is_same

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T, class U > struct is_same; |  | (since C++11)

If `T` and `U` name the same type (taking into account const/volatile qualifications), provides the member constant value equal to true. Otherwise value is false.

Commutativity is satisfied, i.e. for any two types `T` and `U`, is_same<T, U>::value == true if and only if is_same<U, T>::value == true.

If the program adds specializations for `std::is_same` or `std::is_same_v`(since C++17), the behavior is undefined.

### Helper variable template

template< class T, class U > constexpr bool is_same_v = is_same<T, U>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` and `U` are the same type, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Possible implementation

    template<class T, class U>
    struct is_same : std::false_type {};

    template<class T>
    struct is_same<T, T> : std::true_type {};

### Example

Run this code

    #include <cstdint>
    #include <iostream>
    #include <type_traits>

    int main()
    {
        std::cout << std::boolalpha;

        // some implementation-defined facts

        // usually true if 'int' is 32 bit
        std::cout << std::is_same<int, std::int32_t>::value << ' '; // maybe true
        // possibly true if ILP64 data model is used
        std::cout << std::is_same<int, std::int64_t>::value << ' '; // maybe false

        // same tests as above, except using C++17's std::is_same_v<T, U> format
        std::cout << std::is_same_v<int, std::int32_t> << ' ';  // maybe true
        std::cout << std::is_same_v<int, std::int64_t> << '\n'; // maybe false

        // compare the types of a couple variables
        long double num1 = 1.0;
        long double num2 = 2.0;
        static_assert( std::is_same_v<decltype(num1), decltype(num2)> == true );

        // 'float' is never an integral type
        static_assert( std::is_same<float, std::int32_t>::value == false );

        // 'int' is implicitly 'signed'
        static_assert( std::is_same_v<int, int> == true );
        static_assert( std::is_same_v<int, unsigned int> == false );
        static_assert( std::is_same_v<int, signed int> == true );

        // unlike other types, 'char' is neither 'unsigned' nor 'signed'
        static_assert( std::is_same_v<char, char> == true );
        static_assert( std::is_same_v<char, unsigned char> == false );
        static_assert( std::is_same_v<char, signed char> == false );

        // const-qualified type T is not same as non-const T
        static_assert( !std::is_same<const int, int>() );
    }

Possible output:

    true false true false

### See also

same_as (C++20) | specifies that a type is the same as another type (concept)
`decltype` specifier(C++11) | obtains the type of an expression or an entity

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